Arginine is a semi-essential amino acid that is required during periods of maximal growth, severe stress, and injury. Arginine is a substrate for protein synthesis but also modulates cellular biochemical functions via conversion to a number of biologically active compounds. Arginine is utilized by a vast variety of metabolic pathways that produce a variety of biologically active compounds such as nitric oxide, creatine phosphate, agmatine, polyamines, ornithine, and citrulline. Arginine supply is primarily regulated by two enzyme systems: arginase (part of the urea cycle) and nitric oxide synthase. Arginine has many effects in the body that include modulation of immune function, wound healing, hormone secretion, vascular tone, insulin sensitivity, and endothelial function. Arginine mediates its effects via nitric oxide independent and dependent pathways. Nitric oxide modulates many cellular functions that include vascular tone, expression of adhesion molecules, leukocyte adhesion, and platelet aggregation. Arginine modulates the development of atherosclerotic cardiovascular disease, improves immune function in healthy and ill patients, stimulates wound healing in healthy and ill patients, and modulates carcinogenesis and tumor growth. Thus, arginine is a biologically active dietary compound with numerous physiologic and pharmacological activities.